1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Super-heterodyne receivers traditionally receive an RF signal that must be converted to base band by way of an intermediate frequency (IF). Thereafter, the IF signal is amplified and filtered to define a communication channel. In a transmitter, similarly, a base band signal is up converted to the intermediate frequency wherein the amplification and subsequent filtering are carried out at the IF stages. While some systems skip the IF conversion step, wideband systems typically require conversion to IF stages. Depending on the signal bandwidth and the type of communication system, semiconductor devices are not yet able to allow full integration of active filters operating at the elevated intermediate frequencies for a wideband or high data rate communication network. To carry out filtering at the intermediate frequencies, surface acoustic wave filters (SAW) are commonly used. The SAW filters have the drawback, however, of being bulky, heavy and expensive. Additionally, the SAW filters require low impedance matching thereby resulting in high power consumption. Because they are often powered by battery, portable wireless communication devices are not readily adaptable for such systems in that they are required to be inexpensive, light and consume lower amounts of power. Thus, there is a need to design transceiver systems that eliminate the use of intermediate frequency filters.
An alternate approach to using a higher intermediate frequency that requires the SAW filters is to convert the RF signal to an intermediate frequency that is sufficiently low to allow the integration of on-chip channel selection filters. For example, some narrow band or low data rate systems, such as Bluetooth, use this low intermediate frequency design approach.
One problem using low intermediate frequencies, however, is satisfying image rejection requirements for the systems. The image rejection requirement for the down conversion is hard to meet and is usually limited to about −40 dB. Thus, this low intermediate frequency approach is limited for narrow band or low data rate systems. Wide band or high data rate systems require an intermediate frequency that is not low enough for the integration of channel selection filters given the technology that is available today for semiconductor processes. There is a need, therefore, for a wireless transceiver system that allows for full integration on-chip of circuit designs that support high data rate and wideband communications.